Python: How to connect shapekeys via drivers?
The line skey_driver = key.driver_add('value') will create a new driver, any existing driver will be replaced by the new driver.

You can delete an existing driver before creating a new one using key.driver_remove()

Note that replacing the driver with a new one will leave the driver variables behind, you can remove driver variables with skey_driver.driver.variables.remove(var) or all of them with -

for v in skey_driver.driver.variables:
    skey_driver.driver.variables.remove(v)